You need a root account in Linux for this! I used Ubuntu 10.04. The SDK is for Windows as well, but there's no readme for the files. So I set up VMWare with Ubuntu so I can use Linux inside Windows.

For Linux:
In Ubuntu, Fedora, Yellow Dog, etc, type "sudo apt-get install yum" in Terminal.
Type in your root password.
Now, look for a repository with PS3 binaries. You can get some from PS3Bodega.
You need the PS3 SDK kit as well, the full one. You can't get the full 600MB one from pretty much anywhere unless you search. I can't give a link since it's not allowed.
With yum and the RPM installed, now you can just type "yum ps3unpkgr" in Terminal instead of code editing.
When you get the SDK and install the binaries (the RPM is right there), you can use "ps3unpkgr -x <PKG name>" and unpack everything.

To pack it into a PKG again, use "ps3unpkgr -c <PKG name>" and it will pack.
I'm not going to sit and explain everything so thoroughly if you don't know how to do it. If you really are wanting more, reach me another way.

To do this in Windows:
Download the PS3 SDK.
Go into the "host-win32" folder.
Go into "bin."
In Command Prompt, drag "make_package_npdrm.exe" into the prompt, and use -x <PKG name> to decompress. Use -c <PKG name> to compress.
This unpacker will only work for certain PKGs! Luckily, the patch falls under this category.